The most popular way to sleep is favored by more than four out of 10 people, especially among women, who are twice as likely as men to sleep curled up on their side. For the most part, it's a healthy way to doze, because it allows your spine to rest in its natural alignment. The fetal position might also help ward off conditions like Alzheimer's or Parkinson's: Recent research on animals suggests that your brain does a better job of clearing waste that can lead to these neurological diseases when you're sleeping on your side rather than on your back or stomach. This position's also good for pregnant women. You should stick to lying on your left side while expecting. It improves circulation to your growing baby and prevents your uterus from pressing against your liver.